When you're new to cooking, preparing chicken might make you nervous. If it's overcooked the flesh will be relatively dry and flavorless, but if it's undercooked the chicken presents a food safety risk. It can be especially disconcerting if blood appears to be running out, along with the cooking juices. Fortunately, it's not as scary as it seems. Red or pink in the cooking juices is entirely normal, and food safety can be assured by using a thermometer.
The Juices
Most recipes instruct you to bake chicken "until the juices run clear." Those juices are usually pink while the chicken is cooking, but it's not because of blood. The muscle fibers in chickens, like other birds and animals, rely on a substance called myoglobin to carry oxygen to their cells. In the heat of the oven, those muscle fibers begin to contract as they cook. This means they can't hold as much liquid, and -- like squeezing a sponge -- some of the juices from the meat will begin to leak out. The myoglobin in the juices contains iron, which gives them a pink color when they leak out.
Running Clear
As the chicken continues to bake, the heat disrupts the chemical reaction that causes the myoglobin to appear pink. The juices cooking out from the chicken will lose their rosy hue, becoming clear. At that point they're a sort of extra-pure chicken broth. If you insert an instant-read thermometer into the thickest portion of the chicken at this stage, it should show a food-safe temperature of 165 degrees Fahrenheit or above. It's entirely possible your chicken can reach this temperature and still run blood or show a red color near the bone. These are also normal characteristics of modern chickens.
Running Blood
If the juices running from your chicken contain dark red streaks, as opposed to the pink of myoglobin, that's blood. It usually runs from the ends of the leg bones, in chicken legs or a whole roast chicken. The chicken's leg bones make a semiporous container for the bone marrow, where blood is manufactured. During cooking, the blood heats and expands, and forces itself out of the bone. If a thermometer shows your chicken to be at a food safe temperature, this isn't a concern.
Red at the Bone
It's also possible for properly cooked chicken to appear red, or even bleed, at the thigh bone. The femoral artery, which runs along the thigh bone, carries blood through the chicken's leg. Even after cooking, it might contain some dark red blood. It's unsightly, but not a food safety risk. It's also common for properly cooked chicken, especially young fryers, to be a deep pink or even red at the bone. This is because pigments in the bone marrow seep through the thin bones of these juvenile birds, tinting the meat with a heat-stable pigment. Again, this is not a food safety concern as long as your chicken has been cooked to a food safe temperature of 165 F.
